1. Field of the Invention
The present invention relates to stepper motor controls generally and, more particularly, but not by way of limitation, to an improved stepper motor control that increases drive voltage as step rate increases, while maintaining frame temperature of the motor relatively constant, and that produces very low levels of electromagnetic interference (EMI) and radio frequency interference (RFI).
2. Background Art
Stepper motors suffer from a well-known problem in that, as step rate increases, the impedance of the motor windings and the generated back EMF increases, causing current and performance to drop off. Some attempts to address this and related problems are discussed in the following patent documents:
U.S. Pat. No. 3,368,128, issued Feb. 6, 1968, to Parrish, and titled STEP MOTOR CONTROL CIRCUIT INCLUDING A VOLTAGE CONTROLLED OSCILLATOR, describes a closed loop stepper motor control system which allows a stepper motor "to control its own velocity and/or power according to the dictates of its load."
U.S. Pat. No. 3,424,961, issued Jan. 28, 1969, to Leenhouts, and titled LOAD RESPONSIVE, STEPPING MOTOR SPEED CONTROL CIRCUIT, describes a stepper motor control which reduces motor speed (step rate) with increasing load, which then allows for an increase in phase currents.
U.S. Pat. No. 3,444,447, issued May 13, 1969, to Newell, and titled MULTI-PHASE STEP MOTOR CONTROL CIRCUITS INCLUDING MEANS FOR SUPPLEMENTING THE NORMAL ENERGIZATION OF THE WINDINGS, describes a stepper motor control which addresses the above problem by using an energy storage technique during switched-off phase periods (unipolar motors) and a multi-phase overlapping technique.
U.S. Pat. No. 3,452,263, issued Jun. 24, 1969, to Newell, and titled STEP MOTOR DRIVE SYSTEM INCLUDING CURRENT FEEDBACK, describes a stepper motor control which uses closed loop current monitoring and a power wasting series resistor (to limit slow speed current) for unipolar motors.
U.S. Pat. No. 3,505,579, issued Apr. 7, 1970, to Leenhouts et al., and titled POWER SUPPLY FOR A STEPPING MOTOR, addresses the problem by using "an inductance means that is positioned in the A.C. part of the power circuit" to control phase currents in unipolar motors.
U.S. Pat. No. 3,662,245, issued May 9, 1972, to Newell, and titled CONTROL CIRCUIT FOR ENERGIZING THE WINDINGS OF MULTI-PHASE STEP MOTORS INCLUDING A TWO LEVEL SUPPLY VOLTAGE, describes a stepper motor control which employs a closed loop type unipolar driver which has a power wasting series, current limiting, resistor with a means of selectively bypassing it at "preselected time increments".
U.S. Pat. No. 3,665,284, issued May 23, 1972, to Loyzim, and titled POWER SUPPLY FOR A STEPPING MOTOR DEPENDENT UPON VOLTAGE LEVEL AND PULSING RATE, describes a stepper motor control which is similar to the '579 patent above, for unipolar motors, and which controls the A.C. supply for an extended speed range (3,000 to 5,000 steps/second).
U.S. Pat. No. 4,684,865, issued Aug. 4, 1987, to Auger, and titled METHOD AND DEVICE FOR CONTROLLING THE ACCELERATION OF AN ELECTRIC STEPPING MOTOR, describes a stepper motor control which uses a complex microprocessor-based system to provide the fastest acceleration and deceleration ramps for stepper motors.
U.S. Pat. No. 4,788,484, issued Nov. 29, 1988, to Bolash et al., and titled METHOD AND APPARATUS FOR DRIVING A STEPPER MOTOR WITH MULTIPLE VOLTAGES, describes a stepper motor control which is microprocessor-based and which provides two discrete voltage levels for each energy pulse supplied to a stepper motor.
U.S. Pat. No. 5,034,674, issued Jul. 23, 1991, to Sato, and titled MOTOR DRIVING VOLTAGE CONTROL DEVICE, describes a stepper motor control which employs a microprocessor-based system to provide discrete voltage levels and duration for each energy pulse to reduce battery power consumption.
U.S. Pat. No. 5,648,710, issued Jul. 15, 1997, to Ikeda, and titled SYSTEM FOR CONTROLLING DRIVE OF STEPPING MOTOR, also describes a stepper motor control which employs a microprocessor-based system which is a pulse-width-modulation (chopper) constant current type drive with features to prevent "erroneous operation".
The foregoing control systems either do not adequately address the problem, are expensive, are complicated, and/or address entirely different problems.
Accordingly, it is a principal object of the present invention to provide a stepper motor control that increases voltage to the stepper motor as the step rate increases, providing a corresponding increase in motor performance.
It is a further object of the invention to provide such a stepper motor control that increases voltage to the stepper motor while maintaining a relatively constant frame temperature rise.
It is an additional object of the invention to provide such a stepper motor control that is simple and economically implemented.
Another object of the invention is to provide such a stepper motor control that produces very low levels of EMI/RFI.
It is yet a further object of the invention to provide such a stepper motor control that can provide over-current protection of the motor windings during a locked rotor condition.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.